Single‐Step Synthesis of An Ideal Chain Antiferromagnet [H ₂ (4,4′‐bipyridyl)](H ₃ O) ₂ Fe ₂ F ₁₀ with Spin <i>S</i> =5/2

Abstract One‐dimensional (1D) magnets are of great interest owing to their intriguing quantum phenomena and potential application in quantum computing. We successfully synthesized an ideal antiferromagnetic spin S =5/2 chain compound [H 2 (4,4′‐bpy)](H 3 O) 2 Fe 2 F 10 (4,4′‐bpy=4,4′‐bipyridyl) 1 , using a single‐step low‐temperature hydrothermal method under conditions that favors the protonation of the bulky bidentate ligand 4,4′‐bpy. Compound 1 consists of well‐separated (Fe 3+ −F−) ∞ chains with a large Fe−F−Fe angle of 174.8°. Both magnetic susceptibility and specific heat measurements show that 1 does not undergo a magnetic long‐range ordering down to 0.5 K, despite the strong Fe−F−Fe intrachain spin exchange J with J / k B =−16.2(1) K. This indicates a negligibly weak interchain spin exchange J ′. The J ′/ J value estimated for 1 is extremely small (&lt;2.8×10 −6 ), smaller than those reported for all other S =5/2 chain magnets. Our hydrothermal synthesis incorporates both [H 2 (4,4′‐bpy)] 2+ and (H 3 O) + cations into the crystal lattice with numerous hydrogen bonds, hence effectively separating the (Fe 3+ −F−) ∞ spin chains. This single‐step hydrothermal synthesis under conditions favoring the protonation of bulky bidentate ligands offers an effective synthetic strategy to prepare well‐separated 1D spin chain systems of magnetic ions with various spin values.